1. Field of the Invention
The present apparatus relates to filtering liquids which contain particulate matter.
2. State of the Art
Water often contains particulate material which must be removed before the water can be used. For example, many industrial processes require water which has only minimal quantities of particulate matter. One conventional method for removing particulate material from water is to pass the water downwardly through a bed of filter media comprising sand or some other granular material while removing the filtered water from beneath the filter bed. As the process is carried out particulate material accumulates in the bed thus resulting in a loss of hydraulic head across the bed. After a substantial quantity of particles builds up in the bed the head loss becomes so great that it is necessary to stop the filtering process and remove the deposited particles. Generally the particles are removed by passing streams of air and water upwardly through the bed thereby suspending the media, causing separation of the media particles, and carrying deposited particles upwardly and thence to disposal. This process is called backwashing.
Backwashing a filter bed often is carried out in two steps; first the bed is air backwashed and then it is water backwashed. The air backwashing step loosens and separates the particles of the filter bed, and the water backwashing step fluidizes the bed and carries deposited particles upward and from the bed. In both steps fluid flow through the bed must be uniform across the breadth of the bed. If the flow is not uniform, in areas of high velocity the flow will cause particles of the filter media to be carried upward and lost to disposal along with the deposited particles. Whereas, in areas of low velocity, little or no backwashing will occur. Also, filter beds are often composed of layers of different particulate materials, and non-uniform backwashing velocity can result in mixing of the layers.
One conventional device for backwashing a filter bed is taught in U.S. Pat. No. 3,954,620. This patented device comprises a perforated floor which supports the filter bed and a plurality of horizontally disposed conduits arranged below the floor. A plurality of nozzles are affixed beneath the floor and in communication with the conduits. Connected to the conduits is a means for introducing backwashing air and water into the conduits so that the air and water will flow through the nozzles and upwardly through the perforations in the floor and thence into the filter bed. In this conventional device the conduits, the nozzles, and the floor are arranged relative to one another so that two plenum chambers are formed, one between the conduits and the nozzles and one between the nozzles and the floor. In this system the nozzles, which are beneath the perforated floor, do not contact the filter bed. During air backwashing one cushion of air is formed in each of the two plenum chambers. The cushions of air aid in equalizing the air pressure across the filter bed; however, such a two-cushion system, with only one cushion of air preceding the nozzles, has proved to be difficult and expensive to construct and is believed inadequate to give uniform air distribution.